Level is likely not at steady state but trending upwards (average half-life: ~22 hrs, but can range from 7 to 42 hrs due to saturation kinetics. Time to steady state can range from 3 to 50 days)

Using IBW (as patient is obese), maintenance dose should be 365-512 mg/day

Drug interactions with phenytoin (Lexicomp):
– Divalproex may ↓ the protein binding of phenytoin (as both compete for binding to albumin). This may lead to an initial ↑ in free phenytoin and to a ↓ in total phenytoin concentrations. With long-term concurrent use, total phenytoin concentrations may ↑.
– Phenytoin may ↓ the serum concentration of divalproex. There is also a potential for hepatotoxicity due to the ↑ concentration of a hepatotoxic valproic metabolite if phenytoin dose is increased.
– Phenytoin may ↓ the concentration of paliperidone and trazodone
– Trazodone may ↑ the concentration of phenytoin

As divalproex dose is being increased to pre-admission dose, total phenytoin levels may not accurately indicate that patient is on a therapeutic dose.

P: Suggest

Continue current phenytoin dose, and monitor patient closely for seizures